The accident at the Chernobyl Nuclear Power Plant (ChNPP) in northern Ukraine on 26 April 1986 was followed by a phase of cleanup and recovery, which officially lasted until the end of 1990. This effort involved a large number of clean-up workers, the so-called “liquidators.” About 400,000 liquidators took part in cleanup and recovery activities within the 30-km zone around the ChNPP in 1986–1987 (the years of largest radiation hazard and highest intensity of cleanup operations); most of them were from Ukraine, the Russian Federation and Belarus (WHO 2006
). The Chernobyl liquidators were mainly exposed to external whole-body photon (i.e. gamma and x-rays) doses that depended primarily on their work locations and on the time after the accident when the work was performed. Unfortunately, for a number of reasons, individual doses were monitored inadequately or were not monitored at all for the majority of liquidators (Chumak 2007
). There were also problems with the registration and archival of the results of dosimetric monitoring. Therefore, the available estimates of individual dose are insufficient in terms of both scope and quality. At the same time, demand for reliable dosimetric information is high because of the need for a realistic assessment of the radiation doses received by this population group and requests from studies of health effects of the Chernobyl accident.
On-going epidemiological studies on Chernobyl cleanup workers critically depend on the availability of unbiased and accurate individual dose estimates for all study subjects. The International Agency for Research on Cancer (IARC) initiated two case-control studies of (a) hematological malignancies and of (b) thyroid cancer to evaluate the radiation-induced risk of these diseases among the Chernobyl liquidators. The primary goal of these studies is an evaluation of the effects of protracted exposure and radiation type in the low-to-medium dose range (0–500 mGy). The study population comprises liquidators from the Baltic countries (~15,000), Belarus (~66,000), and the Russian Federation (~65,000), who worked in 1986–1987 and who were either registered in the Chernobyl registries (in Belarus and Russia) or included in established cohorts (in Estonia, Latvia and Lithuania). A total of 70 patients with hematological malignancies, 107 with thyroid carcinoma, and 710 control subjects were included in the main data set (Kesminiene et al. 2008
). The subjects of the hematological study were all males, while 38% of the subjects of the thyroid study were females.
In addition, a collaborative case-control study of leukemia and other blood diseases in Ukrainian male liquidators is being conducted by the Research Center for Radiation Medicine of the Academy of Medical Sciences of Ukraine (RCRM) and the U.S. National Cancer Institute (NCI) (Romanenko et al. 2008a
). The study population includes approximately 110,000 persons who worked as liquidators between 1986 and 1990. For the first analysis, a total of 71 leukemia cases and 501 controls were identified as study subjects (Romanenko et al. 2008a
, in press).
To satisfy epidemiologic study requirements, individual dose estimates were needed for all subjects (some of whom are deceased) included in the three studies. The main requirements for dosimetry of the study subjects are (i) uniform quality of dose estimates, to be achieved via application of a single universal method of dose assessment, (ii) ability to estimate doses for all of the dose levels, and (iii) applicability of dose-estimation process to all subjects, including those deceased. The third requirement is particularly significant for the leukemia studies, as this disease has large probability of lethality and many of the cases had already died before the study began.
None of the available methods of retrospective dosimetry or the existing dosimetric data sets could meet all these stringent criteria. The dose values recorded in the state registries of liquidators are called official dose records (ODR). In the state registries of Belarus, Ukraine, and Russia, ODR are available for only about 20, 50, and 60% of registered liquidators, respectively (UNSCEAR 2000
). In the Baltic States, the ODR were drawn from official documents confirming participation in the clean-up activities and are available for 82% of the members of the Chernobyl liquidator cohorts in Estonia and Latvia, and for 69% in Lithuania (Rahu et al. 2006
; Kesminiene et al. 1997
). Further, as was demonstrated elsewhere (Chumak et al. 2007a
), the ODR are usually biased upward. Advanced retrospective biodosimetry techniques like EPR (Electron Paramagnetic Resonance) dosimetry with tooth enamel (Chumak et al. 1999
) or FISH (Fluorescence In Situ Hybridization) applied to lymphocyte chromosomes (Edwards 2000
) are limited by labor intensive analysis, insufficient availability of samples (EPR), or inadequate sensitivity threshold (FISH). Both techniques are only applicable to live subjects; post mortem
extraction of teeth for EPR analysis is feasible in principle but, for ethical reasons, it is not considered in epidemiologic studies in which a large number of samples are required. Besides, with both techniques, it is impossible to separate the Chernobyl exposure from any other unknown pre- or post-Chernobyl exposure (occupational, natural, dental, or medical). In addition, leukemia patients are frequently treated by methods that cause chromosome translocations and hence may invalidate the results of FISH.
In the Chernobyl context an alternative method, called analytical dose reconstruction (ADR), was developed soon after the accident by the Institute of Biophysics‡‡‡
in Moscow for the retrospective assessment of doses received by ChNPP personnel during the first days after the event (Illichev et al. 1996
). The work histories of the ChNPP staff were reconstructed on the basis of free-flow interrogation, the results of which had to be confirmed by eye witnesses. A conservative ‘radiation protection’ philosophy was used to estimate the doses. The method used a time and motion approach in which both the dose rates and the exposure times tended to be overestimated. Fuzzy-logic algebra was used for uncertainty propagation (Kryuchkov et al. 1998
). An independent verification of the ADR dose estimates by EPR dosimetry demonstrated, as expected, that the ADR doses were overestimated by a factor of at least two (Chumak et al. 2005
). In addition, the application of the ADR method was limited to the highly skilled and knowledgeable personnel of the ChNPP who were able to recall and describe their actions and movements in detail, and whose work histories could be confirmed.
It was suggested that a modified version of the ADR approach could be applied to a broader population of liquidators. Four main revisions were considered: (i) use of ‘realistic’ rather than conservative parameter values in the dose-estimation process, (ii) simplification of the interview process and use of a standard validated questionnaire, (iii) development of software that allows description of movements of a person with the help of a geographic map, and (iv) application of stochastic simulation for the assessment of uncertainties. The result of this modification was named RADRUE–Realistic Analytical Dose Reconstruction with Uncertainty Estimation. Description of the formalism of the RADRUE methodology and discussion of particular solutions and features of the computer-aided dose calculations are presented in this paper.